In an HVDC transmission, each one of the converters usually consists of two series-connected six-pulse bridges. Each bridge is connected to the alternating-voltage network via a separate winding transformer. The transformers of the bridges (or the valve windings of a common transformer) are designed with different connections (usually star and delta connections, respectively) in such a way that the alternating voltages of the bridges are subjected to a 30.degree. phase shift and the converter hence becomes a twelve-pulse converter. HVDC transmissions of this kind are amply described in the literature, for example in Brich Uhlmann: "Power Transmission by Direct Current", Spring-Verlag Berlin Heidelberg New York 1975 (see, e.g., FIG. 2.7, p. 15, or FIG. B.1, p. 187). Since the converter bridges are connected to the alternating-voltage network via transformers, a possibility of technical-economic optimization of the direct voltage and direct current levels of the transmission is obtained. Since the converter bridges are connected to the alternating-voltage network via separate winding transformers, a galvanic separation is obtained between the bridges and the alternating-voltage network. This means that, in the manner described above, two converter bridges can be d.c.-series-connected and thus that higher resultant pulse numbers and a reduction of the harmonic content (theoretically an elimination of the lowest harmonics) can be obtained. In this way, the amount of filter equipment can be reduced, which is important since the cost of the filter equipment constitutes an important part of the total cost of a typical HVDC plant. The galvanic separation also means that a converter cannot generate a direct current in the alternating-voltage network, which would involve a risk of certain disturbances, such as transformer saturation.
The advantages of the type of converter station described above have caused it to become practically universally prevailing in HVDC plants.
In a dissertation entitled "HGU-Kurzkupplung ohne Transformatoren", by Dipl.-Ing. Knut Gebhardt, Technische Hochschule Darmstadt, 1976/1977, it has been proposed to connect the converters in a transformerless manner in a back-to-back connection. In this dissertation, the above-described conventional connection is shown in FIG. 1, page 4, and examples of a transformerless connection are shown in FIG. 2, page 5. At first sight, the transformerless connection is economically advantageous since the relatively high cost of the converter transformers is eliminated. However, the connection has several disadvantages, which have caused the connection not to be used to any significant degree in practice. First of all, the direct-voltage level of the plant is determined by the voltage in the alternating-voltage networks, which means that there is no possibility of optimization of the d.c. link and the converters with respect to voltage and current. Secondly, a transformerless HVDC plant is limited to six-pulse operation of the converters, which results in the existence of harmonics of low orders (5 and 7), which necessitate considerably more costly equipment for harmonic filtering. Thirdly, in a plant of this kind, third tone currents (harmonic currents of the orders 3, 9, 15, 21 . . . ) are generated on the d.c. side of a converter. These currents flow out into the alternating-voltage network of the converter. In this network, the currents are of zero-sequence type and give rise to considerable drawbacks in the form of telecommunication disturbance and voltage distortion in the network. In less strong alternating-voltage networks, the voltage distortion becomes such a serious disadvantage that the transformerless connection cannot be used without taking special steps.
However, it is, of course, possible to arrange filters for the above-mentioned third-tone currents. Such a filter can thus be arrange don the a.c. side of a converter. However, the filter is large and expensive, and it has proved difficult to avoid resonance effects between the filter and the alternating-voltage network. Alternatively, a third-tone filter can be arranged in the form of a suppression filter on the d.c. side of the converter. Also in this case, the dimensions and costs of the filter equipment are high, and a considerable risk of resonance effects arises. These facts contribute to the transformerless connection only being considered possible in connection with strong alternating-voltage networks.